In recent years, various forms of rear wheel steering apparatuses, each of which is a portion of a four-wheel steering (4WS) system for a vehicle, have been disclosed. As disclosed in JP 5098242 (Reference 1), the rear wheel steering apparatus is basically made up of “a rod that is connected to rear wheels of a vehicle; a housing that supports the rod, and is fixed to a chassis of the vehicle; and a motor that is accommodated in the housing, and drives the rod such that the rear wheels are turned”. The apparatus is provided with a speed reduction mechanism that converts the rotational motion of the motor into the linear motion of the rod, and transmits the converted motion, and a planetary gear mechanism is used as an example of the speed reduction mechanism.
In the planetary gear mechanism disclosed in Reference 1, a metal flat head pin is used as a member that rotatably supports a planetary gear (illustrated in FIG. 2 of Reference 1). The flat head pin, a flat head part of which serves as a retainer holding the planetary gear in position in an axial direction, is commercially available; however, a general-use flat head pin for use in the rear wheel steering apparatus cannot be found, and a dedicatedly designed flat head pin is required to be prepared, thereby causing an increase in cost. A configuration, in which a locking member made of synthetic resin is joined to a tip end portion of a circular columnar metal pin for general use, is deemed as a countermeasure against this problem; however, the locking member with a simple disk shape adsorbs grease coated on the planetary gear, and co-rotates with the planetary gear. As a result, frictional heat is produced in a joint portion between the pin and the locking member, and the function of the locking member retaining the planetary gear becomes deteriorated, which is a problem.